This game has a separate US release and a EU release, and the info in the dat isn't very clear which is which but the first entry is not both. Filtering for US in ClrMamePro lands you with the "Europe" ROM, since it's marked as USA.
Title/region need corrected on these entries.

Also, possibly both should be titled "Daffy Duck - The Marvin Missions" depending on if you go by the title screen, or if by box/cart art then current title is correct.

I'm guessing then "The Marvin Missions" is how it should be titled if going by title screen, and judging by how the Taz-Mania confusion was handled:
"Taz-Mania (USA, Europe)" is the game titled Taz-Mania consistently in the US, but in Europe titled "Taz-Mania" in-game and "Tazmanian Devil in Island Chase" (additionally Looney Tunes 2) on the box. (though the database is incorrect about the publisher, this one should be Sunsoft, not T*HQ)
"Taz-Mania (Europe)" is "Taz-Mania" in Europe and "Taz-Mania 2" in the US, and this is the one published by THQ.

If GameFAQs is right (which is a risky thing to say), the THQ game might have preceeded the Sunsoft game in Europe. (which is very odd, considering it uses the same engine as another crappy 1993 license game We're Back: A Dinosaur's Story in the US and something like four or five license variations/name changes in EU/AU. They really shopped that thing around. )

Pretty sure I changed Daffy Duck. The original DoM data had the EU release marked as NA, but I actually had the cartridge and could clear up the confusion. Another thing that went on was the verification which of these versions actually was for the SGB.

I'll need to check later if I made a mistake somewhere, but IIRC, the game has "Super Game Boy cartridge" printed on but the game isn't actually SGB-capable. However, there appears to be a separate release that is that also got a European release, but not very much information is available apparently. I'm not sure why "<release name="Daffy Duck (Europe)" region="USA"/>" has region USA. Should I have put "(EUR PARENT)" instead of "(USA PARENT)"? I thought that field meant which region the parent is assigned to?

US Box Art scan has NOA info on the back, and the game ID is there. Makes sense the first entry would be the US one with SGB support being it's the larger of the two(I'm assuming palettes etc, I'm not knowlegeable here).

I have the latter one on my desk right now. It doesn't support Super Game Boy. I think they might have mixed up the rollout of new stickers and the rollout of the actual new game ROMs at some point. Not being able to trust the label on the game makes it so hard to verify which version was originally dumped, apparently.

So when I edited the original info in DoM, I worked on the assumption that there are indeed some European (DMG-YS-NOE-1) games out there that do contain the revision A ROM with SGB support. ** That's why European region info wasn't removed.

* side-note: I just found Personal Organizer has printed "Official Game Boy Source Pak", don't know what the difference is, though. Maybe it's just a goof-up as well.

** I'm aware that the current Dump for the game with SGB support contains "Rev 0" in its internal game header. However, there are indeed a few games where developers forgot to update the game header and it slipped by Nintendo.